System for monitoring the condition of structural elements

ABSTRACT

A system for monitoring the condition of elongate structural elements, for example, railway rails, and a method of designing and manufacturing the system is disclosed. The method includes identifying and selecting suitable modes of propagation and signal frequencies that can be expected to travel large distances through an elongate structural element; designing a transducer that will excite the selected mode at the selected frequency; numerically modelling the transducer as attached to the elongate structural element; validating the transducer design by analysing a harmonic response of the selected mode of propagation to excitation by the transducer, and manufacturing one or more transducers for use in the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of patent application Ser.No. 14/239,666, which is a U.S. national stage application ofInternational Application No. PCT/IB2012/054264 entitled “A System forMonitoring the Condition of Structural Elements and a Method ofDeveloping Such a System”, which has an international filing date ofAug. 23, 2012, and which claims priority to South African PatentApplication No. 2011/06192, filed 23 Aug. 2011.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a system for monitoring the condition ofelongate structural elements and more particularly but not exclusively,to a system for monitoring and detecting cracks and breaks in railwayrails. The invention furthermore extends to the methodology of designingand developing such a system.

Related Art

There are several methods and systems which have been proposed formonitoring the integrity of elongate structural elements, and inparticular railway rails. These methods and systems are aimed atdetecting cracks in the rails before they develop into complete breaks,and also to detect breaks in a railway network where they have alreadyoccurred. If a crack or break in the rail is not detected beforehand, itcould result in the derailment of the railway vehicle travelling on thetrack. It will be appreciated that such derailments cause financial lossand can also result in injury and loss of life. Also, it should be notedthat although reference is made to railways, these systems are equallyapplicable to other applications where lengths of structural steel areutilised, such as for example mine shafts and bridges.

One method of detecting cracks and breaks in the rails of railway tracksis disclosed in South African patent 99/6936, the contents of which isincorporated herein by reference. The method includes the step ofproviding a number of autonomous acoustic transmitter units, and anumber of acoustic receiver units located between the transmitter units.The various units are spaced apart from one another by predetermineddistances. The transmitter units introduce a series of acoustic pulseswith specific frequency composition into the rails and the receiverunits detect and analyse the pulses to monitor any unwanted conditionconcerning the rail. This method requires the use of transmitters andthe use of receivers in order to monitor the condition of the rail.

Development of transducers for this method of detecting and monitoringcracks and breaks in railway rails is discussed in “Development ofpiezoelectric transducers for a railway integrity monitoring system”,Philip W. Loveday, Smart Structures and Materials 2000: Smart Systemsfor Bridges, Structures, and Highways, Proceedings of SPIE Vol. 3988,2000, Newport Beach, pp. 330-338. The system makes use of piezoelectrictransducers which are mounted (clamped) under the crown of the rail onthe outside of the track. The method of clamping the piezoelectrictransducers is described in PCT patent application WO 2004/098974, thecontent of which is incorporated herein by reference.

The piezoelectric transducers are spaced along the length of the railwaynetwork and they periodically transmit ultrasonic waves through therails. The waves propagate through the track from one transducer towardsa downstream transducer which acts as a receiving station. Typically,the transducers are spaced about 1 km apart. If the ultrasonic signal isnot detected at the receiver station, the receiver station activates analarm indicating that the rail either has a crack or is broken.

A disadvantage associated with the above system is that thepiezoelectric transducers are attached (clamped) under the crown of therail on the outside of the track. The piezoelectric transducers arelarge and cannot be attached under the crown on the inside of the trackbecause they would interfere with the train wheels. The piezoelectrictransducers have to be removed from the rail during routine trackmaintenance because a ‘tamping’ machine used to re-pack the ballastunder the sleepers has wheels that engage the outside of the crown. Theremoval and re-attachment (which requires re-tightening of the clampstwo weeks after re-attachment) of the piezoelectric transducersincreases the maintenance cost of the system and results in periods oftime when the system is inoperable.

In addition, the existing system is not suited for distance in excess of1 km, as the transmitted signal is not strong enough, and because thetransducer is also not accurately matched to the particular structuralelement to which it will be attached from a propagation and operatingfrequency point of view.

The detection systems described above have generally been developedusing design methodologies that do not optimally incorporate the use ofmathematical modelling techniques in which the transducer and railresponse is mathematically modelled, and in which the transducer is thendesigned in an iterative manner. This resulted in the selection oftransducers that are not necessarily optimized for a particularapplication, and which may result in the transducers being larger thanrequired in practice, whilst also not performing optimally insofar astransmission and receiving of signals are concerned.

It is therefore an object of the invention to provide a system formonitoring and detecting cracks and breaks in railway rails that willaddress the disadvantages described above.

It is also an object of the invention to provide a piezoelectrictransducer for use in the system according to the present invention.

It is a further object of the invention to provide a method fordeveloping a transducer-based failure detection system, which will atleast partially overcome the above disadvantages, and which will also bea novel and useful alternative to existing design methodologies.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof developing a transducer-based failure detection system, the methodincluding the steps of:

-   -   identifying modes of propagation and signal frequencies that can        be expected to travel large distances through an elongate        structural element;    -   selecting a suitable mode of propagation and frequency of        operation;    -   designing a transducer that is adapted to excite the selected        mode at the selected frequency;    -   numerically modelling the transducer as attached to the elongate        structural element; and    -   analysing a harmonic response of the selected mode of        propagation to excitation by the transducer in order to validate        the transducer design.

The step of identifying modes of propagation and frequencies that can beexpected to travel large distances through an elongate structuralelement preferably comprises the use of a numerical model of aparticular rail profile having predetermined material properties.

The selection of a suitable mode of propagation and frequency ofoperation preferably entails selecting a mode of propagation having lowattenuation over a large range of frequencies, and which is relativelyinsensitive to small changes in rail profile.

The method may include the further steps of iteratively changingdimensions of transducer components in order to achieve an optimalresponse of the selected mode of propagation at the frequency ofoperation, and computing a predicted displacement time response of therail to an electrical excitation of the transducer.

The method may further include a verification phase including the stepsof:

-   -   manufacturing a prototype in accordance with the modelled        transducer;    -   measuring free electrical transmittance of the transducer, and        comparing the measured free electrical transmittance with        transmittance predicted by the model described above.

The verification phase may also include the steps of:

-   -   attaching the transducer to a predetermined length of the        structural element;    -   measuring a displacement response on a surface of the structural        element; and    -   comparing the measured response to the predicted displacement        time response.

The verification phase may still further include the steps of performingin-use field measurements in order to confirm excitation of the selectedmode, as well as propagation with low attenuation.

According to a second aspect of the invention there is provided a systemfor monitoring and detecting cracks or breaks in rails of a railwaytrack, the system including a plurality of transducers definingtransmitting and receiving stations of the system, characterised in thatthe transducers are preferably located on the inner sides of the rails.

There is provided for the plurality of transducers to be in the form ofa series of single transducers located at predetermined spaced apartpositions, with ultrasonic waves periodically being transmitted alongthe rail from one transducer used as a transmitter to a next transducerused as a receiver.

There is also provided for the plurality of transducers to be in theform of a series of single transducers spaced apart at predeterminedintervals, with ultrasonic waves periodically being transmitted alongthe rail from one transducer used as a transmitter, and reflected by acrack in the rail to the same transducer, which is also used as areceiver.

There is further provided for a plurality of transducers to be locatedat each predetermined position so as to define an array of transducers.A number of arrays may be provided, with the arrays of transducersspaced apart at predetermined intervals.

In one embodiment the transducers are permanently attached to the railson the inner sides of the rails.

Preferably, the rails include a web and a crown, and there is providedfor the transducers to be attached underneath the crown, oralternatively to the web of the rails.

Advantageously, the transducers are of a geometrical size, shape andconfiguration enabling the attachment thereof to the rails withoutinterfering with a wheel of a railway vehicle travelling on the rails.

In one embodiment the system is configured such that an upstreamtransducer transmits an ultrasonic wave along the rail which is receivedby a downstream transducer if there are no cracks or breaks in the rail.The system is furthermore configured such that if the downstreamtransducer does not receive the ultrasonic wave transmitted by theupstream transducer, an alarm is triggered, warning of the possiblepresence of a crack or break in the rail.

In another embodiment the system is configured such that a transducertransmits and ultrasonic wave along the rail, and the same transducerreceives the ultrasonic wave if it is reflected by a crack in the rail.The system is furthermore configured such that if the transducerreceives the reflected ultrasonic wave, an alarm is triggered, warningof the possible presence of a crack in the rail.

In a still further embodiment the system comprises both thefunctionalities described above.

In one embodiment, the transducers are spaced apart by distances ofabout 1 to 3 kilometres. Preferably, the transducers are spaced apart bydistances of about 2 kilometres.

Preferably, the transducer is a piezoelectric transducer.

According to another aspect of the invention there is provided atransducer suitable for use in a system for monitoring and detectingcracks or breaks in rails of a railway track, the system including aplurality of transducers defining transmitting and receiving stations ofthe system, characterised in that the transducers are located on theinner sides of the rails.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of anon-limiting example, and with reference to the accompanying drawing inwhich:

FIG. 1 shows a system in accordance with one embodiment of the presentinvention, the system including two piezoelectric transducers which areattached to the rails of the railway track, for monitoring and detectingcracks or breaks in the rails;

FIG. 2 shows the output of an initial modelling process used to selectan appropriate mode of propagation and operating frequency for aparticular rail profile; and

FIG. 3 shows the experimental comparison between the performance of aprior art system and the system in accordance with the invention.

EXAMPLE OF DESIGN METHODOLOGY

The methodology and development procedure used to develop atransducer-based failure detection system in accordance with theinvention is described with reference to FIG. 2. The method is acomputer implemented method.

1. Analysis of Dispersion of Rail Profile on Damped Support.

This step involved developing a numerical (semi-analytical finiteelement method) model of the rail profile that also incorporated thematerial properties of the rail. The development of semi-analyticalfinite element models is a methodology known in the art, but which hasnot heretofore been applied in this particular application. The modelwas analysed to determine which modes of propagation and frequenciescould be expected to travel large distances. Some modes of propagationand frequencies that were expected to travel with low attenuation areindicated by the arrows in FIG. 2. The size of the dots represents theexpected propagation performance. The dots form curves describingdifferent modes of propagation. The arrows indicate three modes thatcould be suitable and it was accordingly decided to use a signal with afrequency centred at the arrow location.

2. Selection of Appropriate Mode of Propagation and Frequency.

Based on the results from step 1 a mode of propagation and frequency ofoperation were selected. The selected mode had low attenuation over areasonably large range of frequencies so that it could be expected towork over a range of temperatures. This analysis is a qualitativeprocedure in which modes and frequencies with the lowest relativeattenuation were considered. The analysis did not attempt to quantifythe actual attenuation. Any person skilled in the art will be able tounderstand and correctly apply this qualitative approach. In essence, ifthe system is required to detect a particular type of crack the selectedmode of propagation should contain energy in the region where the cracksoccur. The mode of propagation and range of frequencies was chosen to berelatively insensitive to changes in the rail geometry due to forexample rail profile grinding or changes in the axial load in the rail.In this particular example, a mode with wavenumber of 82 rad/m at 35 kHzwas selected, and additional analyses were performed to ensure that theselected point was insensitive to rail grinding, temperature changes andaxial load.

3. Conceptual Design of Transducer Configuration.

A transducer configuration suitable for permanent attachment to a railwas subsequently conceptualized. In this example, a sandwich-typetransducer suitable for being attached under a crown of the rail wasdesigned. The transducer design was not fundamentally different instructure and configuration to existing transducer designs, but wasexpected to be better matched with the system as a whole due to theintegrated design methodology.

4. Numerical Modelling of Transducer Configuration Attached to Rail andSizing to Achieve Large Response at Required Frequency.

A numerical model (3-D finite element method) of the piezoelectrictransducer was prepared, and was coupled to the numerical model(semi-analytical finite element method) of the rail. The harmonicresponse of the selected mode to electrical excitation of the transducerwas subsequently analysed. The dimensions of the transducer componentswere then iteratively changed in order to achieve an optimal response ofthe selected mode at the operating frequency. This methodology waspreviously developed by the inventor, and is described in more detail in“Simulation of Piezoelectric Excitation of Guided Waves Using WaveguideFinite Elements”, Loveday P W, IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency control; vol. 54 no. 10; October 2007, thecontents of which is incorporated herein by reference. Finally, thepredicted displacement time response of the rail due to tone-burstelectrical excitation of the transducer was determined for use in alater verification phase. This methodology was also previously developedby the inventor, and is described in more detail in “Analysis ofPiezoelectric Ultrasonic Transducers Attached to Waveguides UsingWaveguide Finite Elements”, Loveday P W, IEEE Transactions onUltrasonics, Ferroelectrics, and Frequency control; vol. 55 no. 9;September 2008, the contents of which is incorporated herein byreference.

5. Transducer Prototype Manufacture and Measurement in Lab.

Based on the above modelling, a number of prototype transducers weremanufactured. The free electrical admittance of each transducer wasmeasured and compared with modelled predictions to verify correctmanufacture. A transducer was subsequently attached to a short raillength in a lab environment and electrical tone-burst excitation wasapplied thereto. The displacement response on the rail surface at adistance of 1 m or more was measured using a laser vibrometer. Themeasured results were then compared to the predicted displacement timeresponse from step 4.

6. Field Measurements to Confirm Transducer Performance and PropagationMode in Rail.

The transducer was subsequently attached to an actual rail in the field,and was driven electrically. Scanning laser vibrometer measurements wereperformed on the rail surface at different distances from the transducer(e.g. 5 m, 300 m, 500 m). Modes present in the measured data wereextracted to confirm that the selected mode was being excited and thatit does indeed propagate with low attenuation. Long-rangetransmit-receive measurements were performed with the new transducersand compared to the same measurements performed with the prior arttransducers.

7. Industrialization of Transducer.

Subsequent to the transducer design process described above, thetransducer was industrialised, which process included the preparation ofmanufacturing data packs and qualification and acceptance testprocedures.

The above process yielded an optimised transducer design, which meetsthe required design criteria, whilst also being of a relatively smallsize compared to existing transducers used in similar failure detectionapplications.

The design methodology can furthermore be used in the optimised designof transducers that are application and profile specific, and willtherefore result in more accurate design of transducers for use infailure-detection systems.

DESCRIPTION OF AN EMBODIMENT OF THE SYSTEM

The relative small size of the transducer designed using the abovedesign methodology enables the use of a new configuration, which is nowgenerically described in more detail with reference to FIG. 1.

Typically, railway tracks include two parallel rails 11 that are mountedon sleepers 12. The rails 11 typically have a profile including a base13 which rests on the sleepers 12, a web 14 extending upwardly from thebase 13, and a crown 15 extending transversely from the web 14, on whichthe wheels 16 of a railway vehicle travel. It will however beappreciated that the system of the present invention, withmodifications, can be used on any rail profile. It will be appreciatedthat the described embodiment relates to one particular use in a railwayapplication, but that the system can likewise be utilised in anyapplication involving lengths of structural steel, for example bridgesand mine shafts.

In accordance with the present invention, the system 10 includestransducers 17 for detecting cracks and breaks in the rails. Thetransducers used in the present system are piezoelectric transducers 17.The piezoelectric transducers 17 can be permanently attached underneaththe crown 15 of the rails, or attached to the web 14 of the rails 11.The piezoelectric transducers 17 are of such a geometrical size, shapeand configuration that they can be attached to the rails 11 withoutinterfering with the wheels 16 of the railway vehicle utilising therails 11. In the preferred embodiment of the invention, thesepiezoelectric transducers 17 are located on the rails 11 on the innersides of the rails 11.

The piezoelectric transducers 17 transmit ultrasonic waves which travelalong the rails 11, and also operate as receivers for receiving theultrasonic waves transmitted along the rails 11. These piezoelectrictransducers 17 periodically transmit ultrasonic waves along the rails 11to monitor the condition of the rails 11 i.e. to detect cracks andbreaks in the rails 11.

The piezoelectric transducers 17 are spaced apart from one another atpredetermined distances along the rails 11. Typically, the piezoelectrictransducers 17 are spaced apart from one another by distances of about 1to 3 kilometres.

The system 10 is configured such that a transducer 17 located upstreamon the rail 11 transmits a signal in the form of an ultrasonic wavealong the rail 11, which is received by a transducer 17 locateddownstream of the upstream transducer 17. If the ultrasonic wavetransmitted by the upstream transducer 17 is received by the downstreamtransducer 17, the system 10 determines that there are no cracks orbreaks in the rail 11. However, if the upstream transducer 17 transmitsan ultrasonic wave which does not reach the downstream transducer 17,the system 10 determines that there is a possibility that there is acrack or break in the rail 11.

In the event that the transducer 17 located downstream does not receivethe ultrasonic wave transmitted by the upstream transducer 17, thesystem 10 is configured to generate a signal indicating the possiblepresence of a crack or break in the rail 11. The signal triggers analarm warning of the possible presence of the crack or break in the rail11. The alarm is transmitted to a base station or the railway vehicleutilising the railway track.

In the above example the system is utilised as a signal transmissionsystem. However, in another embodiment (not shown) the same transducerscan also be used in a pulse-echo configuration where the same transducertransmits and receives a signal. The signal is transmitted by thetransducer, and if there is a crack in the rail the signal will bereflected back to the same transducer, which will then also act as thereceiver. The transducers developed using the design methodologydescribed above will also be particularly suitable for this type ofpulse-echo monitoring system due to the enhanced signal strength.

Irrespective of the system configuration (pulse-echo or transmission),an array of transducers (for example 4) can be provided at eachpredetermined location to improve the performance of the system becausethe additional transducers allow better control of the modes to beexcited and transmission in one direction along the rail and receivingfrom one direction.

It will be appreciated that a combination of the transmission andpulse-echo systems would be an optimal solution. This is now possibledue to the new design methodology resulting in transducers that are muchbetter matched to the operating conditions, thus resulting in strongersignal strengths whilst also significantly reducing the size of thetransducers used. In the past, larger transducers with robust designswere used to propagate the waves through the rails. This was due in partto a lack of detailed modelling of the system, and over above thephysical sizes of the transducers, the design methodology used did notallow for optimal signal strength and propagation of such signal throughthe rails. Now, as a result of the methodology described above, thesystem has been optimised and one can more accurately predict theresults of the wave propagation. Surprisingly, as a result of themathematical modelling and experimentation it has been found that thetransducers can be smaller than originally thought, and that the smallertransducers perform better than the older, larger and robusttransducers. As a result of the smaller geometrical size, shape andconfiguration of the transducers, the system is optimised and hasimproved functionality, and in particular addresses the disadvantagesmentioned above.

A comparison between the performance of a prior art system and the newsystem was performed on a particular length of railway track. It wasconcluded that the transmit performance and receive performance of thenew transducers were both 20 dB improved over the prior art transducers.The above is graphically illustrated in FIG. 3. In FIG. 3, the two barson the left hand side of the graph represents the performance of a priorart system secured to two adjacent rails of a railway. The transmissionvoltage was 1300 Vp. The two bars in the middle represent the resultsfrom a combined system where the transmitters of the olds system wereused, whereas the receivers were transducers designed in accordance withthe new design methodology. The transmission voltage was again 1300 Vp.The two bars on the right hand side represent the results of the newsystem—i.e. both the transmitting and receiving transducer were designedusing the new design methodology. In this case the transmission voltagewas however 280 Vp. It will be noted that a 40 dB improvement wasobserved.

As a result of this 40 dB transmit-receive performance, it was foundthat the while the prior art system could only operate at 900 m spacing,on this particular rail section, the new transducers enabled operationat 2000 m spacing.

The system of the present invention addresses the problems discussedabove. Firstly, the need to remove the piezoelectric transducers duringroutine track maintenance and the need to re-attach piezoelectrictransducers after the track maintenance is eliminated. Advantageously,the piezoelectric transducers of the present invention are attachedunder the crown, or attached to the web of the rail on the inner sidesof the rails, and thus there is no need to remove them during routinetrack maintenance. Moreover, the need to re-tighten the clamps after twoweeks of re-attachment, according to the previous system, is eliminated.Secondly, the system performs much better than the prior art system, andcan successfully be implemented for operational distances of 2000 m, onpoor condition rail, where only 900 m was previously possible. This is adirect result of the new design methodology that results in largersignal transmission and improved receive sensitivity.

It will be appreciated that the above is only one embodiment of theinvention and that there may be many variations without departing fromthe spirit and/or the scope of the invention.

What is claimed is:
 1. A system for monitoring and detecting cracks orbreaks in rails of a railway track, the system including a plurality oftransducers defining transmitting and receiving stations of the system,characterised in that the transducers are located on the inner sides ofthe rails.
 2. The system of claim 1 in which the plurality oftransducers is in the form of a series of transducers located atpredetermined spaced apart positions, with at least one transducerprovided at each predetermined position, and with ultrasonic wavesperiodically being transmittable along the rail from one transducer usedas a transmitter to a spaced apart transducer used as a receiver.
 3. Thesystem of claim 2 in which an array of the transducers is located ateach predetermined position.
 4. The system of claim 1 in which theplurality of transducers is in the form of a series of transducerslocated at predetermined spaced apart positions, with at least onetransducer provided at each predetermined position, with ultrasonicwaves periodically being transmittable along the rail from onetransducer used as a transmitter, and reflected by a crack in the railto the same transducer, which is also used as a receiver.
 5. The systemof claim 4 in which an array of the transducers is located at eachpredetermined position.
 6. The system of claim 1, claim 2 or claim 4 inwhich the predetermined spaced apart positions are spaced apart by adistance of about 1 to 3 kilometres.
 7. The system of claim 6 in whichthe predetermined spaced apart positions are spaced apart by a distanceof about 2 kilometres.
 8. The system of claim 6 in which an array of thetransducers is located at each predetermined position.